Dielectric resonators are used in many circuits, particularly microwave circuits, for concentrating electric fields. They can be used to form filters, oscillators, triplexers and other circuits. The higher the dielectric constant of the dielectric material out of which the resonator is formed, the smaller the space within which the electric fields are concentrated. Suitable dielectric materials for fabricating dielectric resonators are available today with dielectric constants ranging from approximately 10 to approximately 150 (relative to air). These dielectric materials generally have a mu (magnetic constant) of 1, i.e., they are transparent to magnetic fields.
FIG. 1 is a perspective view of a typical dielectric resonator of the prior art. As can be seen, the resonator 10 is formed as a cylinder 12 of dielectric material with a circular, longitudinal through hole 14. Individual resonators are commonly called “pucks” in the relevant trades. While dielectric resonators have many uses, their primary use is in connection with microwaves and particularly, in microwave communication systems and networks.
As is well known in the art, dielectric resonators and resonator filters have multiple modes of electrical fields and magnetic fields concentrated at different center frequencies. A mode is a field configuration corresponding to a resonant frequency of the system as determined by Maxwell's equations. In a dielectric resonator, the fundamental resonant mode frequency, i.e., the lowest frequency, is the transverse electric field mode, TE01δ (or TE, hereafter). Typically, it is the fundamental TE mode that is the desired mode of the circuit or system into which the resonator is incorporated. The second mode is commonly termed the hybrid mode, H11δ (or H11 hereafter). The H11 mode is excited from the dielectric resonator, but a considerable amount of electric field lays outside the resonator and, therefore, is strongly affected by the cavity. The H11 mode is the result of an interaction of the dielectric resonator and the cavity within which it is positioned and has two polarizations. The H11 mode field is orthogonal to the TE mode field. There are additional higher modes. Typically, all of the modes other than the mode of interest, e.g., the TE mode, are undesired and constitute interference. The H11 mode, however, typically is the only interference mode of significant concern. The remaining modes usually have substantial frequency separation from the TE mode and thus do not cause significant interference with operation of the system. The H11 mode, however, tends to be rather close in frequency to the TE mode. In addition, as the frequency of the TE mode is tuned, the center frequency of the TE mode and the H11 mode move in opposite directions to each other. Thus, as the TE mode is tuned to increase its center frequency, the center frequency of the H11 mode inherently moves downward and, thus, closer to the TE mode center frequency. By contrast, the third mode, commonly called the H12 mode, not only is sufficiently spaced in frequency from the TE mode so as not to cause significant problems, but, in addition, it moves in the same direction as the TE mode responsive to tuning.
FIG. 2 is a perspective view of a microwave dielectric resonator filter 20 of the prior art employing a plurality of dielectric resonators 10. The resonators 10 are arranged in the cavity 22 of a conductive enclosure 24. The conductive enclosure 24 typically is rectangular, as shown in FIG. 2. Microwave energy is introduced into the cavity via a coupler 28 coupled to a cable, such as a coaxial cable. Conductive separating walls 32 separate the resonators from each other and block (partially or wholly) coupling between physically adjacent resonators 10. Particularly, irises 30 in walls 32 control the coupling between adjacent resonators 10. Walls without irises generally prevent any coupling between adjacent resonators. Walls with irises allow some coupling between adjacent resonators. Conductive adjusting screws may be placed in the irises to further affect the fields of the adjacent resonators and provide adjustability of the coupling between the resonators, but are not shown in the example of FIG. 2. By way of example, the field of resonator 10a couples to the field of resonator 10b through iris 30a, the field of resonator 10b further couples to the field of resonator 10c through iris 30b, and the field of resonator 10c further couples to the field of resonator 10d through iris 30c. Wall 32a, which does not have an iris, prevents the field of resonator 10a from coupling with physically adjacent resonator 10d on the other side of the wall 32a. 
One or more metal plates 42 are attached to the top cover plate (top cover plate not shown) to affect the field of the resonator to set the center frequency of the filter. Particularly, plate 42 may be mounted on a screw 43 passing through top cover plate (not shown) of enclosure 24 that may be rotated to vary the spacing between the plate 42 and the resonator 10 to adjust the center frequency of the resonator. An output coupler 40 is positioned adjacent the last resonator 10d to couple the microwave energy out of the filter 20 and into a coaxial connector (not shown). Signals also may be coupled into and out of a dielectric resonator circuit by other methods, such as microstrips positioned on the bottom surface 44 of the enclosure 24 adjacent the resonators. The sizes of the resonator pucks 10, their relative spacing, the number of pucks, the size of the cavity 22, and the size of the irises 30 all need to be precisely controlled to set the desired center wavelength of the filter and the bandwidth of the filter. More specifically, the bandwidth of the filter is controlled primarily by the amount of coupling of the electric and magnetic fields between the electrically adjacent resonators. Generally, the closer the resonators are to each other, the more coupling between them and the wider the bandwidth of the filter. On the other hand, the center frequency of the filter is controlled in large part by the size of the resonators themselves and the size and spacing of the conductive plates 42 from the corresponding resonators 10. Generally the larger the resonator, the lower its center frequency may be.
Prior art dielectric resonator filters have limited frequency bandwidth performance. The maximum frequencies at which they can perform effectively is typically limited to about 55 to 60 GHz. The effective bandwidth range of prior art dielectric resonator filters is typically on the order of 3 to 20 MHz. In particular, the bandwidth is restricted because the couplings between resonators are limited.
Prior art resonators and the circuits made from them have many drawbacks. For instance, as a result of the positions of the fields of the resonators, prior art resonators have limited ability to couple with other resonators (or with other microwave devices such as loop couplers and microstrips). That is why filters made from prior art resonators have limited bandwidth range. Further, prior art dielectric resonator circuits such as the filter shown in FIG. 2 suffer from poor quality factor, Q, due to the presence of separating walls and coupling screws. Q essentially is an efficiency rating of the system and, more particularly, is the ratio of stored energy to lost energy in the system. The fields generated by the resonators pass through all of the conductive components of the system, such as the enclosure 20, plates 42 internal walls 32 and 34, and adjusting screws 43 and inherently generate currents in those conductive elements. Those currents essentially comprise energy that is lost to the system.
Furthermore, the volume and configuration of the conductive enclosure 24, substantially affects the operation of the system. The enclosure minimizes radiative loss. However, it also has a substantial effect on the center frequency of the TE mode. Accordingly, not only must the enclosure be constructed of a conductive material, but it must be very precisely machined to achieve the desired center frequency performance, thus adding complexity and expense to the fabrication of the system. Even with very precise machining, the design can easily be marginal and fail specification.
Even further and perhaps most importantly, prior art resonators have poor mode separation between the desired TE mode and the undesired H11 mode.
FIGS. 3A and 3B illustrate magnitude of the electric fields for the TE and H11 modes, respectively, in a typical prior art cylindrical resonator 10. As shown, the Electric Field 31 of the TE mode is circular, oriented transverse of the cylindrical puck 12, and is concentrated around the circumference of the resonator 10, with some of the field inside the resonator and some of the field outside the resonator. A portion of the field should be outside the resonator for purposes of coupling between the resonator and other microwave devices (e.g., other resonators or input/output couplers). If all of the field is concentrated inside the dielectric resonator, it would be very difficult to control the coupling between resonators.
The electric field of the H11 mode is orthogonal to the TE mode. The electric field 33 forms a circle around the puck 10 parallel to the page and is concentrated near the surface. It is very difficult to physically separate the H11 mode from the TE mode. Accordingly, methods for suppressing the H11 mode have been developed in the prior art. For instance, metal strips 41 such as illustrated in FIG. 4 have been placed on the surface of the resonators to suppress the H11 mode by causing its tangential electric field to be zero at the metal strips 41, effectively causing the suppression of the mode because its maximum field strength is located near the metal strips. In practice, while this technique for suppressing the H11 mode is relatively effective in terms of suppressing the H11 mode, it also typically suppresses the TE mode significantly. In theory, the effect on the TE mode should be insignificant, but experiments show that this is not the case in the real world and that this method for H11 suppression actually significantly affects Q for the TE mode. Experiments show that this technique typically might cause losses of about half of the power of the TE mode, thus substantially reducing the Q of the resonator and the overall system in which it is employed.
Accordingly, it is an object of the present invention to provide improved dielectric resonators.
It is another object of the present invention to provide improved dielectric resonator filters and other circuits employing dielectric resonators.
It is a further object of the present invention to provide a method and apparatus by which improved coupling is achieved between dielectric resonators and other devices, such as coupling loops, microstrips and other dielectric resonators.
It is another object of the present invention to provide dielectric resonators and dielectric resonator filters in which the H11 mode is substantially suppressed or eliminated.
It is yet another object of the present invention to provide dielectric resonators and dielectric resonator circuits with improved mode separation between the TE mode and the H11 mode.
It is yet a further object of the present invention to provide dielectric resonators and dielectric resonator circuits that are easily tunable.
It is one more object of the present invention to provide dielectric resonators and dielectric resonator circuits with more effective coupling than in the state of the art.
It is a further object of the present invention to provide dielectric resonators and dielectric resonator filters with improved Q factors.